The present invention relates generally to an optical element having an antireflection film, and more particularly to a structure of an antireflection film used for an optical element in an exposure apparatus. The present invention is suitable, for example, for an optical element used for an illumination optical system and a projection optical system in an exposure apparatus that uses an F2 laser with a wavelength of about 157 nm for a light source.
In order to meet the recent demands for the fine processing, a projection exposure apparatus intends to improve the resolving power through use of the exposure light having a shorter wavelength. Recently, the exposure light source shifts from a KrF excimer laser (with a wavelength of approximately 248 nm) to an ArF excimer laser (with a wavelength of approximately 193 nm), and a development of the F2 laser advances. In addition, since a highly durable or light-resistant optical element is required, an antireflection film or coating should be applied to the substrate to reduce a light amount loss, flare, and ghost, etc. caused by the surface reflections.
One known antireflection method only to the light having a single specific wavelength is a so-called V coat that stacks two layers made of a high refractive index material and a low refractive index material having an optical film thickness λ0/4 from a substrate where λ0 is a designed central wavelength. For example, one known antireflection film alternately layers GdF3 and MgF2. See, for example, Applied Optics, Vol. 41, No. 16, 1 Jun. 2002, pp. 3248-3255, “Development of Optical Coating for 157-nm Lithography. II. Reflectance, Absorption, and scatter measurement.”
However, the exposure light having a short wavelength has such a high light energy that the light resistance of the antireflection film lowers. For example, the antireflection film that uses GdF3 and MgF2 deteriorates and remarkably decreases the transmittance to the F2 laser beams of several tens of mega pulses. As a result of studies of the cause, this inventor has discovered that the conventional antireflection film has such a layered structure of two different crystalline materials that its surface becomes rough, and the high light energy causes organic materials to adhere to the surface of the antireflection film.